When molding articles from rubber or plastic materials within closed molds wherein a blank of rubber composition or plastic composition or a pre-formed partially-shaped rubber or plastic article is placed into the mold cavity of a multi-section mold and the mold then is closed, air usually is entrapped within the mold cavity as the mold is closed. Unless the entrapped air is exhausted from the mold cavity, the entrapped air can prevent the rubber or plastic composition from flowing and fully filling the mold cavity. When this occurs, the molded article does not conform in detail to the desired shape and, depending upon the severity of the defect, must be scrapped or sold as a "blemished" product. Although the problem is not experienced if the pressure within the mold cavity of the closed mold is sufficiently high to force the entrapped air to become absorbed into the rubber or plastic molding composition during the molding operation, in many molding situtations the necessary high pressures are not desirable or cannot be tolerated.
One method which has been used to overcome the problem involves drilling a number of small vent holes (commonly about 2 millimeters in diameter) through the walls of the mold at those areas of the mold cavity at which air pockets normally are formed as the mold is closed. In the use of such vent holes, as the rubber or plastic composition flows to fill the mold cavity, the entrapped air is forced ahead of the flowing rubber or plastic composition and is forced to exit from the mold cavity through the vent holes. The rubber or plastic composition flows into the vent holes producing rubber or plastic vent hole plugs which form a part of the surface of the molded product. The initial cost of drilling the numerous vent holes in the mold is substantial. Also, some of the plugs of rubber or plastic formed in the vent holes may break from the molded article as the article is separted from the mold leaving the plugs lodged within the vent holes, making it necessary periodically to re-drill the vent holes to remove the plugs. The vent hole plugs which remain as part of the surface of the molded article are unsightly and for aesthetic considerations are removed, usually by cutting, from the article thereby adding expense to the manufacturing cost.
U.S. Pat. No. 3,989,430 describes some disadvantages and problems resulting from the formation of the vent hole plugs during the molding of rubber tires and suggests inserting nails into the vent holes which (nails) have nail heads that have an irregular undersurface to allow air to escape into the vent holes but prevent rubber composition from flowing into the vent holes to form plugs. The procedure suggested does not eliminate the necessity of drilling the vent holes in the tire mold initially, however, and raises the possibility of nails being pulled accidentally from the vent holes when the tire is removed from the mold, particularly after the mold has been in use for a long period of time.